The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzle repair thereof.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor to generate hot combustion gases which are discharged through a turbine for extracting energy therefrom.
The turbine typically includes multiple stages having a stationary turbine nozzle preferentially directing the combustion gases through a downstream row of rotor blades.
Each turbine nozzle includes a row of turbine stator vanes integrally joined to radially outer and inner bands. The bands are typically segmented circumferentially with one or more vanes per nozzle segment.
Since the nozzle segments are directly exposed to hot combustion gases during operation, they are typically formed of cobalt or nickel based superalloy metal for ensuring a long useful life. The superalloy is typically cast using advanced casting processes that control the metallurgical properties of the material, including directionally solidified or single crystal compositions.
Additional protection may be provided by coating the nozzle for corrosion resistance and thermal insulation. And, the nozzle typically includes dedicated cooling circuits using cooling air bled from the compressor.
Nevertheless, during prolonged use in the gas turbine engine the nozzles will oxidize, corrode, crack, or otherwise wear, and are therefore subject to periodic maintenance inspections to discover the onset of such wear and perform remedial repair as warranted.
Maintenance repair may include the complete replacement of worn nozzle segments by new segments, or the repair of portions thereof for minimizing maintenance costs. Because nozzle segments are complex in design, are made of expensive materials, and are expensive to manufacture, it is generally more desirable to repair them whenever possible instead of replacing them.
Existing repair processes include techniques such as crack repair and dimensional restoration of airfoil surfaces. However, such existing repairs are limited by local distortion and under-minimum wall thicknesses, which can be exceeded as a result of repeated repair and chemical stripping processes.
Thus, nozzle segments may become damaged to the point where they cannot be repaired by known repair processes. The thermal and mechanical stresses in integrally cast nozzle segments are such that it often occurs that the inner band is repairable while other nozzle segment structure is non-repairable.
Accordingly, it would be desirable to have a method for salvaging the repairable portion of the nozzle segment to avoid scrapping the entire nozzle segment in such a situation.